During the prolonged period that many organic pollutants react in soil, they appear to become sequestered and their behavior changes. This sequestration or aging affects their susceptibility to biodegradation, extractability, and toxicity. However, the means presently do not exist to predict the extent of change in bioavailability with time for any organic compound. Hence, a study is proposed (1) to determine the properties of organic compounds and of soils that govern how aging-in soil affects bioavailability in terms of extractability, biodegradability, and toxicity; and (2) to establish the feasibility of using biodegradation and extraction methods to predict toxicity of organic compounds in soil. Plant (seed germination, root elongation), microbial (nitrification, Ames Salmonella mutant reversion), and invertebrate (acute toxicity to Eisenia foetida) toxicity tests will be employed as a suite applicable to soil-borne hazardous wastes. In addition, studies will be conducted on the mechanisms by which aging occurs, with attention to both porous and non-porous media (synthetic and natural), soil properties, and pore size, tortuosity, and distribution. Emphasis will be on some 25 14C-labeled compounds for which property- outcomes relationships and shifts in known rate and extent of biodegradability may be related to measures of toxicity and sequestration.